Tag: Irving Weissman

How do we get stem cells to differentiate into the cell types we want? Implanting undifferentiated stem cells into a living organism can sometimes result in cells that differentiate into unwanted cell types. Such a phenomenon is called heterotropic differentiation and it is a genuine concern of regenerative medicine. What is a clinical researcher to do? Answer: make a road map of the events that drive cells to differentiate into specific cell types and their respective precursors.

Researchers in the laboratory of Irving Weissman at Stanford University Researchers at the Stanford University School of Medicine have mapped out the bifurcating lineage choices that lead from pluripotency to 12 human mesodermal lineages, including bone, muscle, and heart. The experiments also defined the sets of biological and chemical signals necessary to quickly and efficiently direct pluripotent stem cells to differentiate into pure populations of any of 12 cell types. This is certainly a remarkable paper in many aspects, since Weissman and his group defined the extrinsic signals that control each binary lineage decision that occur during stem cell differentiation. This knowledge enables any lab to successfully block differentiation toward unwanted cell fates and rapidly steer pluripotent stem cells toward largely pure human mesodermal lineages at most of these differentiation branchpoints.

The ability to make pure populations of these cells within days rather than the weeks or months is one of the Holy Grails of regenerative medicine. Such abilities can, potentially, allow researchers and clinicians to make new beating heart cells to repair damage after a heart attack, or cartilage for osteoarthritic knees or hips, or bone to reinvigorate broken bones or malfunctioning joints, or heal from accidental or surgical trauma.

The Weissman study also highlights those key, but short-lived, patterns of gene expression that occur during human early embryonic segmentation. By mapping stepwise chromatin and single-cell gene expression changes during the somite segmentation stage of mesodermal development, the Weissman group discovered a previously unobservable human embryonic event transiently marked by expression of the HOPX gene. It turns out that these decisions made during human development rely on processes that are evolutionarily conserved among many animals. These insights may also lead to a better understanding of how congenital defects occur.

“Regenerative medicine relies on the ability to turn pluripotent human stem cells into specialized tissue stem cells that can engraft and function in patients,” said Irving Weissman of Stanford. “It took us years to be able to isolate blood-forming and brain-forming stem cells. Here we used our knowledge of the developmental biology of many other animal models to provide the positive and negative signaling factors to guide the developmental choices of these tissue and organ stem cells. Within five to nine days we can generate virtually all the pure cell populations that we need.”

All in all, this roadmap enables scientists to navigate mesodermal development to produce transplantable, human tissue progenitors, and uncover developmental processes.

Even though many stem cells researchers do not believe that they exist, Very Small Embryonic-Like (VSELs) Stem Cells from bone marrow have continued to be a subject of research. A recent paper in the journal Stem Cells and Development by Russ Taichman (University of Michigan) and colleagues has documented, for this first time, that VSELs from mice and humans are multipotent (that is, able to differentiate into several different mature, adult cell types) when transplanted into a living animal.

VSELs are very rare, very small embryonic-like stem cells. They represent a rare population in the bone marrow (less than 0.02% of nucleated cells), but have been identified in most tissues that have been examined, including blood and other solid organs (see Kucia M, J Ratajczak, R Reca, A Janowska-Wieczorek and MZ Ratajczak. (2004). Tissue-specific muscle, neural and liver stem/progenitor cells reside in the bone marrow, respond to an SDF-1 gradient and are mobilized into peripheral blood during stress and tissue injury. Blood Cells Mol Dis 32:52–57).

Because VSELs express some of the same genes that are expressed in human embryonic stem cells (e.g., Oct4, nanog, and stage-specific embryonic antigen or SSEA-1), there is a hope that they can differentiate into cell types from all three embryonic germ layers.

When Taichman and his colleagues implanted human VSELs into mice that had suffer skull injuries, the cells produced robust mineralized tissue that came from human cells. When this mineralized material was examined in more detail, it was clear that the human VSELS had formed neurons, adipocytes (fat cells), chondrocytes (cartilage cells), and osteoblasts (bone cells) within the skull lesions.

The ability of these most-primitive, multipotent stem cells to differentiate into bone, neurons, connective tissue, and other cell types, was also accompanied by a protocol that contained the proper criteria for identifying and isolating VSELs. A second paper that tends to corroborate Taichman’s work was also published in the same volume of Stem Cells and Development.

Malwina Suszynska and others from the University of Louisville, KY, and Pomeranian Medical University (Szczecin) and Jagiellonian University (Krakow), Poland explores the challenges involved with isolating these rare stem cells and the importance of not confusing VSELs with other types of stem cells. In their article, “The Proper Criteria for Identification and Sorting of Very Small Embryonic-Like Stem Cells (VSELs), and Some Nomenclature Issues,” the authors present the most current descriptions and terminology for characterizing VSELs.

“I find the data presented by the Taichman group to be compelling and challenging. However, the current debate as to the significance of the body of publications concerning VSELs can only be resolved by a cooperative investigation across laboratories using identical methodologies and source materials,” says Editor-in-Chief Graham C. Parker, PhD, The Carman and Ann Adams Department of Pediatrics, Wayne State University School of Medicine, Detroit, MI.

I will reiterate my original suggestions: Researchers from the Weissman and Alt laboratories should visit these other laboratories and learn how to isolate VSELs from the people who actually do it. If they cannot get these protocols to work on their own laboratories, or if there is evidence of impropriety or incompetence, then skepticism of the existence of VSELs will certainly be vindicated. However, if these techniques can be learned and made to work in other laboratories, then I think we should be comfortable with the existence of VSELs.

Bone injuries and bone diseases sometimes require bone grafts for proper treatment. In order to find bone for implantation, orthopedic surgeons often take bone from other locations in the body, use bone from cadavers or synthetic compounds that promote the formation of new bone. Bone grafting is a complex surgical procedure and even though it can replace missing bone, it poses a significant health risk to the patient, and sometimes completely fails to foster proper healing.

Bone has the ability to regenerate, but it requires very small fracture space or some sort of scaffold in order to make new bone. Bone grafts can provide that scaffold. A bone graft can be “autologous,” which simply means that the bone is harvested from the patient’s own body (often from the iliac crest), or the graft can be an allograft, which consists of cadaveric bone usually obtained from a bone bank. Finally, synthetic bone grafts are made from hydroxyapatite or some other naturally occurring, biocompatible substance such as Bioglass, tricalcium phosphate, or calcium sulfate.

Making natural bone from stem cells is one of the goals of regenerative medicine, and work from Irving I. Weissman at Stanford University has shown that this hope is certainly feasible.

Weissman and his colleagues evaluated the ability of embryonic stem cells and induced pluripotent stem cells to form bone in a culture environment known to induce bone formation in most circumstances. This culture system (known as an osteogenic microniche) consisted of a scaffold made of poly – L-lactate coated with hydroxyapatite and stuffed with a growth factor called bone morphogen protein-2 (BMP-2). BMP-2 is a known inducer of bone formation and this scaffold is placed inside the bone of a laboratory animal that has suffered a fracture.

After implanting pluripotent stem cells into these osteogenic microniches, they were very pleasantly surprised to find that both embryonic stem cells and induced pluripotent stem cells embedded themselves into the scaffold and differentiated into bone making cells (osteoblasts). They also made new bone and did so without forming any tumors.

These results suggest that local signals from the implanted scaffold and the genera environment within the bone directed the cells to survive and differentiate into osteoblasts. Thus pluripotent stem cells may have the clinical capacity to regenerate bone, which would, potentially preclude the need for risky bone grafting procedures.